The present invention relates to a sliding member of graphite-boron carbides, which is useful as the member in the part where friction is caused by sliding in bearings, cylinders, slide valves and the like, and more in detail, the present invention relates to a novel sliding member showing a favorable specific properties, particularly to friction and abrasion at a high temperature, which is suitably fitted for the use under such conditions that it is difficult to use a fluid lubricant such as lubricating oils and greases, namely the so-called dry friction conditions.
In recent years, with the development of mechatronics, the demands for the sliding members, which are excellent in frictional- and abrasional properties, particularly in the high temperature region have been raised.
For instance, to the sliding members used in a high temperature region of 200.degree. to 500.degree. C., any fluid lubricant such as lubricating oil and grease cannot be applied between the surfaces contacting to each other and accordingly, a solid lubricant such as graphite and molybdenum disulfide is used.
However, since the solid lubricants usually have no fluidity nor wettability as in the fluid lubricants, such a solid lubricant cannot be applied by a means such as painting, impregnating and pouring.
Accordingly, in the case of a solid lubricant, it must be applied by means of (i) covering the friction surface with the solid lubricant, (ii) preparing the sliding member itself of the solid lubricant or (iii) using the mixture of the lubricant and another material. In the case of adopting any of the above-mentioned means, since the lubricating effect is exhibited at the first time after a part of the solid lubricant thus applied on the sliding, friction surface is transferred to the surface of the mating material, the frictional- and abrasional property of the solid lubricant is influenced by the degree of film-forming property of the solid lubricant on the surface of the mating material.
Although graphite which has been broadly used as a solid lubricant is an excellent solid lubricant from the view point that it is excellent in lubricating property, non-melting property, thermal conductivity and heat-resistance and it is small in thermal expansion, it is insufficient to transfer to the surface of the mating material and its film-forming property on the surface of the mating material is insufficient. Accordingly, graphite is not necessarily sufficient in the frictional- and abrasional properties, and in addition, since graphite is low in mechanical strength, there is a difficult point that in the case where the sliding member itself is made from graphite, the periphery of the member must be reinforced by a metal or graphite must be reinforced by impregnating with a metal.
Besides, as the material for sliding members used under the dry conditions, graphite impregnated with a synthetic resin, mixtures of graphite and molybdenum disulfide or polytetrafluoroethylene, and graphite covered with molybdenum disulfide or polytetrafluoroethylene have been proposed, however, since such materials melt, thermally decompose or are oxidized at a high temperature, there is a demerit that they cannot be practically used under the temperature condition of higher than 300.degree. C.
Formerly, the present inventors proposed a process for producing a high-density graphite-boron carbide composite material, the process comprising the steps of adding boron carbide as a sintering accelerator into powdery coke and sintering the thus obtained mixture at a temperature of higher than 2000.degree. C. under a pressure of higher than 200 kg/cm.sup.2 (refer to Japanese Patent Publication No. 56-37192 (1981)).
Japanese Patent, Publication No.,56-37192 (1981) discloses a process for producing the graphite-boron carbide composite material having a high density, for instance, bulk density of 2.07, 2.15 and 2.23, respectively, a high strength and a high hardness, comprising, concretely, the steps of adding 10, 20 and 40% by weight of powdery boron carbide, respectively to powdery pitch coke, sufficiently blending the two components, getting each 25 g of the thus prepared mixture in each of graphite molds of 30 mm in diameter, sintering each mold containing the mixture at 2200.degree. C. under a pressure of 200 kg/cm.sup.2 and after maintaining thereof at the temperature for 1 hour, cooling thereof to room temperature.
As a result of the present inventors' further studies for a process for producing a graphite-boron carbide composite material of a high density, they have found that in the case where sliding members are prepared by using a composite graphite-boron carbide material produced by the steps of mixing by stirring from 65 to 95% by weight of powdery carbon of 10 to 150 .mu.m in an average particle size and from 5 to 35% by weight of powdery boron carbide of 0.5 to 2.5 .mu.m in an average particle size until the powdery boron carbide is uniformly dispersed in the powdery carbon and sintering the thus prepared mixture at a temperature of 2020.degree. C. or more and less than a transition point at which the structure of carbon and boron carbide in the thus prepared mixture is remarkably changed, under a pressure of 100 to 250 kg/cm.sup.2, thereby sintering thereof, the above-mentioned demerits of the sliding members can be solved, and based on the finding, the present invention has been attained.
Namely, the object of the present invention is to provide with the sliding members which overcome the demerits of the conventional solid lubricants, show favorable transferring property to and film-forming property on the surface of the mating material and at the same time, show an excellent frictional- and abrasional characteristic properties and a high mechanical strength in a high temperature region.